There is a semiconductor device such as a metal oxide semiconductor field effect transistor (MOSFET) that has a field plate structure having a field plate electrode provided below a gate electrode. By disposing the field plate electrode, electric field intensity in a drift region of the MOSFET is relaxed. Thus, a breakdown withstand voltage (hereinafter simply referred to as a withstand voltage) between the drain and the source of the MOSFET is improved. Further, due to the improvement of the withstand voltage, the carrier density of the drift region can be set to a high level, and thus, an ON resistance of the MOSFET can be decreased. In other words, according to the MOSFET having the field plate structure, a trade-off relationship between the improvement of the withstand voltage and the decrease of the ON resistance in the MOSFET can be improved.
However, in such a MOSFET, the electric field is concentrated in the p-n junction part between the base region and the drift region, or the vicinity of the lower end of the field plate electrode, and thus, the sufficient withstand voltage cannot be obtained in some cases. An improvement is required for the trade-off relationship between the improvement of the withstand voltage and the decrease of the ON resistance in the MOSFET having the field plate structure.